7.11.2.2 Pump protection functions
7.11.2.2.1 P2t shutdown
Image 7.53: P2t shutdown
P 2851[7; 8] - HYD_Pump can be used to set the critical speed and critical duration.
At the nominal pressure, the critical speed should not be fallen below for a time
longer than the critical duration. The square of the pressure is included in the
calculation here, i.e. at half the pressure, four times the duration will be tolerated.
Reason: Leakage losses have an approximately quadratic relationship to the
pressure.
MOOG
ID No.: CB40859-001 Date: 11/2020
MSD Servo Drive - Device Help
177
7 Control
Meanwhile, the speed is taken into account linearly, i.e. at half the speed, half the
duration will be tolerated.
Reason: The speed / the volumetric flow rate proportional to the speed results in a
proportional heat removal.
Usually, the pump needs to be protected against continuous operation with built-up
pressure and low speeds. To this end, a critical minimum speed n_crit can be
specified so that, at nominal pressure p_N, it will only be allowed to fall below this
speed for a critical duration of t_crit. The following critical limit integral is calculated
continuously in the servo drive for this purpose:
l
i_G = 1 / τ_crit ∙ ∫ ((p/p_N )² - n / n_crit) dt
This takes into account the power loss that has an approximately quadratic
relationship to the pressure, as well as the heat removal that is proportional to the
speed. If the nominal pressure were requested in a stopped state (n = 0), for
example, i_G would increase to a value of 1.0 before t_crit would elapse. At half the
critical speed, twice that time would be required. Meanwhile, a different pressure
would have a quadratic effect, meaning that half the nominal pressure would be
allowed to be present four times the critical duration time.
This function can be used to open a bypass circuit when a configurable limit (i_G =
0.9) is reached. Normally, this will result in the pump speed increasing and, after a
certain hysteresis (i_G = 0.8) is fallen below, the bypass will be closed again in order
to reduce the power loss. If i_G increases all the way to 1.0 instead, an error will be
triggered and the drive will be stopped in order to protect the pump.
Example
Critical speed: 500 bar, Nominal pressure: 100 bar, Critical time: 10 s.
l
This pump can be run at 500 rpm, 100 bar as long as necessary.
l
At very low speeds close to a full stop, and with a pressure of 100 bar, it
would only be permissible to run the pump for 10 s. After 9 s, the bypass
valve (if any) would be opened. If this did not result in the speed increasing,
